B cells constitute an integral part of the immune system. B cell deficiency is one of the major causes of primary immunodeficiency diseases. Of more than 120 known PIDs, B cell deficiencies and combined B and T cell deficiencies are responsible for nearly 40 of them. B cell development is a sequential process consisting of the following stages: hematopoietic stem cell, multilineage progenitor, common lymphoid progenitor, pre-pro-B (fraction A), pro-B (fractions B and C), early pre-B (fraction C'), late pre-B (fraction ), immature B (fraction E), and mature B (fraction F). The process is precisely regulated by coordinated functioning of multiple genes, especially those encoding transcription factors. Sox4 is a gene that encodes a transcription factor and is known to be vital for many developmental processes. Sox4 conventional knockout causes embryos to die due to cardiac deficiency. Fetal liver cells isolated from Sox4 knockout mice fail to generate B cells in recipient mice after transplantation, indicating that the function of Sox4 is indispensable for B cell development. The lethality of the Sox4 conventional knockout has hindered further studies on the role of Sox4 in B cell development. We systematically investigated the role of Sox4 in B cell development using conditional knockout mouse models with Sox4 deletion at different developmental stages. Sox4 deletion in hematopoietic stem cells results in a nearly complete lack of pro-B cells and later stage B cells, whereas Sox4 deletion in pre-B cells or mature B cells has no detectable effect. Although the conditional knockout models have allowed us to reveal the developmental stage-specific functions of Sox4, the lack of B cells has imposed an obstacle for future in-depth studies. To overcome this problem, we have developed an in vitro culture system in which a stage specific developmental event is dependent on the presence of Sox4. This functional system is ideal for characterizing the molecular mechanisms underlying Sox4 function. We have performed gene expression microarray profiling using cultured cells and identified potential Sox4-regulated genes. To pursue in-depth characterization of Sox4 function in B cell development using our novel in vitro system, we propose two specific aims for this project. Aim 1 is to identify and characterize genes directly regulated by Sox4. Aim 2 is to characterize the roles of two downstream targets of Sox4 in early B cell development. Successful completion of this proposed research will elucidate the transcription program regulated by Sox4, advance our understanding of B cell development, and expand our knowledge of PIDs.